1. Field of the Invention
As is well known to all those skilled in this art, both calcium and sulfur are nutrients essential for the culture and/or propagatIon of most living plants and are generally classified by agronomists as secondary nutrients; with nitrogen, phosphorus, and potassium comprising the primary plant nutrients. It is also well recognized that such secondary plant nutrients, to wit, calcium and sulfur, are required for many major and economically important agricultural commodities including, for example, peanuts, as well as most cereals and fiber-producing crops.
The mineral limestone, comprising calcium carbonate (pure or impure). is found in great abundance and, when processed to fine particle-size distribution by, for example, grinding, is used extensively as a liming agent to aid in the adjustment of soil acidity (pH) to thereby effect the maximum production of most agricultural food and fiber crops. It is estimated that, in the United States, some 20 to 25 million tons of agricultural limestone is used annually. Larger-sized (-6 +16 Tyler Standard Screen Scale) limestone is also used as a filler in the production of numerous bulk-blended fertilizers.
The mineral gypsum, CaSO.sub.4 .multidot.2H.sub.2 O, is found extensively in natural massive forms and also is produced in large quantities as a by-product when limestone and sulfuric acid are reacted to produce carbon dioxide (sulfogypsum) or when phosphate rock and sulfuric acid are reacted to produce wet-process phosphoric acid for use in the manufacture of phosphatic fertilizers (phosphogypsum). Gypsum is also used in the manufacture of Portland cement to prevent too-rapid setting and in road construction as a soil stabilizer. As used herein, finely-divided gypsum is either mined and comminuted, or otherwise obtained as a manmade by-product and used at a size distribution somewhat finer than agricultural limestone.
It is estimated that currently, about one-million short tons of gypsum is used annually in the southeastern United States for the cropping of peanuts. This material is applied to the peanut crop at the bloom stage so that when the gynophore "peg" enters the soil, adequate calcium from the gypsum source is readily available for realization of maximum crop yield. In other agricultural areas, large quantities of gypsum are applied to soils which are deficient in sulfur or to other soils to effect adjustments of the undesired salinity thereof.
Agricultural limestone and agricultural gypsum are oftentimes referred to by those skilled in the art as secondary nutrient fertilizers and are usually applied to the intended soil environment in a very finely-divided state. Such fine particle size is required for materials of relatively low solubility to achieve the desired rate of reaction in the soil; therefore, products with a low specific surface area may not be as effective as those applied to the soil as line powder. On the other hand, modern fertilizer-application equipment, designed for broadcast of or for row placement of solid fertilizers, is intended for handling free-flowing granular-type materials such as, for example, granular or pelletized ammonium nitrate or urea. Consequently, numerous problems are encountered when such equipment is used, or more correctly, misused by operators for field placement of such finely-divided material. As specified by USDA, agricultural limestone is of a U.S. Standard Testing Sieve particle-size distribution so that no less than 90 percent passes through the No. 10 mesh screen and no less than 50 percent passes through the No. 60 mesh screen. As shown in Table II and noted in the discussion just thereafter in Example I, infra, the actual size distribution for this material is considerably finer.
2. DescrIption of the Prior Art
The utilization of lignosulfonates, a by-product of the paper pulping industry, is quite widespread and varied running the gambit from fillers for asunder different types of materials to conditioning agents or binders for a variety of products including, for instance, animal feed wherein it also may act as a pelletizing aid as well as a source of metabolizable energy. The Food and Drug Administration apparently has approved same for use in animal feeds in amounts up to 4 percent by weight.
In the embodiment of being utilized as a binding agent, in U.S. Pat. No. 4,587,358, Blouin, May 6, 1986, assigned to the assignee of the present invention, there is disclosed that the utilization of minute amounts, i.e., usually less than about 1 percent and preferably from about 0.4 to about 0.8 percent by weight of lignosulfonates based on the weight of the final product, may be utilized to impart to urea improved storage stability particularly that attribute characterized by a substantial reduction and tendency for caking during storage of freshly prepared urea particles. Blouin teaches that various lignosulfonates may preferably be mixed with the urea melt to ensure a homogeneous mixture prior to the solidification of the resulting product.
In U.S. Pat. No. 4,676,821, Gullett, et al., Jun. 30, 1987, there is taught an improvement evolving from the teachings of Blouin, supra. wherein the product produced by Blouin is utilized in the teachings of Gullett, et al., as substrate for the production of sulfur-coated urea products. According to Gullett, et al., such lignosulfonate-containing substrate results in a sulfur-coated product having a substantially increased time of dissolution, as compared to urea substrate conditioned in the conventional manner with formaldehyde. Accordingly, Gullett, et al., can obtain a target dissolution rate with substantially less sulfur coating utilizing such substrate than with the conventional formaldehyde urea thereby, of course, resulting in a product having a substantially increased nitrogen content.
In U.S. Pat. No. 4,743,289, Mickus, et al., May 10, 1988, assigned to the assignee of the present invention, there is taught that lignosulfonates can be substituted for several materials heretofore utilized as granulation aids during the production of fertilizers produced in size to be eminently effective in the later production of bulk-blended materials. In particular, Mickus, et al., teach that they can substitute several lignosulfonates for the alum that was used as a granulation aid in prior work of two of the coinventors for effecting production of granular crystalline by-product ammonium sulfate. This work of Mickus, et al., evolved from the earlier work of Harrison, et al., taught, described, and claimed in U.S. Pat. No. 4,589,904, May 5, 1986, and assigned to the assignee of the present invention.